Conventional structural assemblies as used in the manufacture of military and commercial aircraft are commonly fabricated using a bonded honeycomb-sandwich construction or a built-up structure. Conventional structural assemblies formed from these types of constructions generally include large numbers of parts and fasteners which can result in extensive tooling and increased labor costs during manufacture and assembly. The component parts of conventional structural assemblies are typically not welded because conventional welding techniques can distort the dimensions and/or shape of the component parts as well as create joints having defects such as porosity, micro-cracking, lack of fusion and poor ductility that can lead to cracking or failure of the joint when subjected to the severe cyclical stresses commonly associated with aeronautical applications.
Additionally, during use, aircraft structural assemblies are subjected to a variety of environmental conditions, temperature variations, severe acoustic and vibration environments, all of which create mechanical and thermal stresses. Over time, the application of cyclical stresses to bonded structural assemblies can lead to disbanding at the joints, and unless repaired, it can result in mechanical failure. Moisture entrapments also can occur during use of the aircraft which in combination with the extreme environmental conditions can result in corrosion which can also weaken the structural assembly. Due to the large number of parts and fasteners utilized in the construction of conventional structural assemblies, maintenance and repair can be time consuming and labor intensive which can be costly over the life of the assembly.
The number of total parts utilized in a bonded honeycomb or built-up structure can also increase the overall weight of the aircraft. Consequently, conventional structural assemblies are generally costly to build and maintain and can adversely affect the weight of the aircraft.
In seeking better structural assembly designs, other types of sandwich structures have been proposed. In particular, one such alternative design is an interlocking design concept such as the Grid-Lock.RTM. interlocking assembly of Tolo Incorporated which is described in U.S. Pat. No. 5,273,806 to Lockshaw et al. and which is shown in FIGS. 1 and 2. An interlocking structural assembly 10 includes first and second machined components 11, 12 which are typically fabricated from aluminum or titanium using a CNC milling machine. The components are machined to include generally planar surface portions 13, 14 having a plurality of integral ribs 15, 16 which extend outwardly from a respective surface portion and coincide with the ribs of the other component. The ribs 15 of the first component 11 are further machined to include grooves 17 for matingly receiving the distal ends of the corresponding ribs 16 of the second component 12. The grooves 17 are precisely machined so as to form a tongue and groove assembly which allows the ribs 15, 16 of the first and second components 11, 12 to be snapped together. Additionally, the interlock ribbing 15, 16 are adhesively bonded together with an adhesive 18 such as epoxy or urethane glue.
Interlocking ribs and grooves require extra stock material. Further, the ribs and grooves have to be machined with specific tolerances in order to obtain a secure fit of the interlocking assembly. The precise machining generally requires extra machining time which can increase the overall manufacturing costs and can result in material waste in cases of operator error.
The use of an adhesive to bond the structural assembly also creates the potential for disbonding of the joints due to degradation of the adhesive from heat exposure or environmental conditions and stress. An adhesive bond failure can accelerate corrosion damage or result in a catastrophic failure of the assembly. Although the use of the interlocking assembly assists the adhesive bond in joining the corresponding ribs, the mechanical strength of the joint remains well below that of the base material.
As a result, there remains a need for structural assemblies which can be manufactured and assembled with a minimum number of parts so as to reduce the costs associated with manufacture, assembly and maintenance of the structural assemblies, as well as to reduce the overall weight of the aircraft. The structural assembly must also be capable of providing high mechanical strength and structural rigidity.